1. Field of the Invention
The present invention relates to oscillators, and more particularly, to oscillators for integrated circuit devices or chips.
2. Description of the Related Art
Oscillators or clocks are often used for a variety of reasons in integrated circuits and computers. For example, in the case of dynamic random access memory (DRAM) integrated circuits, low frequency oscillators are often required in order to generate a refresh clock signal. A DRAM integrated circuit uses the refresh clock signal to refresh its stored data when in a self-refresh mode. Specifically, the refresh clock signal operates to signal the DRAM integrated circuit when the stored data in the DRAM integrated circuit should be refreshed in order to preserve its state.
Generally speaking, low frequency oscillators have conventionally operated by charging a large capacitance value using a small current, then evaluating the voltage level at the large capacitance. When the voltage is determined to exceed a certain level, the state of an output changes which is used to generate a pulse. Conventionally, there are two different and distinct ways to produce the small current that is used to charge the large capacitance. One approach is to provide a large resistor between the capacitance and a supply voltage, and thus produce a small current through the resistor to charge the large capacitance. Another approach is to provide a constant current source to supply a small current to the large capacitance. Each of these approaches is described in additional detail with respect to FIGS. 1 and 2, respectively.
FIG. 1 is a schematic diagram of a first conventional oscillator 100. The first conventional oscillator 100 includes a resistor (R) 102, a capacitor (C) 104, a differential amplifier 106, a reset transistor 108, and an output pulse generator 110. The resistor (R) 102 and the capacitor (C) 104 are connected in series between a supply voltage (V.sub.cc) and ground. A current (i) is produced and flows from the supply voltage (V.sub.cc) to the capacitor (C) 104. The differential amplifier 106 has a first input terminal connected to a node connecting the resistor (R) 102 and the capacitor (C) 104, and a second input terminal connected to a reference voltage (V.sub.REF). The differential amplifier 106 also has an output terminal connected to the output pulse generator 110. The output pulse generator 110 outputs a pulse is generated by the first conventional oscillator 100. The output pulse generator 110 also supplies a reset signal to a gate of the reset transistor 108 to control a discharge operation on the capacitor (C) 104. The combined effect of the charging and the discharging of the capacitor (C) 104 is the production of the periodic pulses (i.e., oscillator or clock) by the output pulse generator 110. One problem with the first conventional oscillator 100 is that the current (i) produced by the resistor (R) 102 decreases as temperature increases because the resistance of the resistor (R) 102 increases with temperature. As a result, the frequency of the periodic pulses undesirably varies with temperature. Another problem with the first conventional oscillator 100 is that the current (i) varies with the supply voltage (V.sub.cc).
FIG. 2 is a schematic diagram of a second conventional oscillator 200. The second conventional oscillator 200 includes transistors 202, 204 and 206, a capacitor (C) 208, a differential amplifier 210, a reset transistor 212, and an output pulse generator 214. The transistors 202 and 206 are p-type transistors, and the transistor 204 is a n-type transistor. The transistors 202, 204 and 206 produce a current (i) that is used to charge the capacitor (C) 208. The transistors 202 and 206 are coupled to a supply potential (V.sub.cc) and form a current mirror arrangement. The current mirror arrangement produces a current (i) that is supplied the capacitor (C) 208. The transistor 204 is activated by a control voltage (V.sub.o) so as to limit the current (i) produced by the current mirror arrangement. The differential amplifier 210 includes a first input terminal connected to the capacitor (C) 208, and a second input terminal connected to a reference voltage (V.sub.REF). The differential amplifier 210 also includes an output terminal for supplying a signal to the output pulse generator 214. The output pulse generator 214 outputs a pulse is generated by the second conventional oscillator 200. The output pulse generator 214 also supplies a reset signal to a gate of the reset transistor 212. When activated, the reset transistor 212 operates to discharge the capacitor (C) 208. The repeated charging and discharging of the capacitor (C) 104 causes the second conventional oscillator 200 to produce periodic pulses (i.e., oscillator or clock). Unlike the first conventional oscillator 100, the second conventional oscillator 200 produces a current (i) that is independent of the supply voltage (V.sub.cc) level. However, one problem with the second conventional oscillator 200 is that the current (i) produced by the current mirror arrangement varies with temperature. As a result, the frequency of the periodic pulses also undesirably varies with temperature.
In general, it is desirable to produce oscillators or clocks that are independent of temperature variations. In the case of a DRAM integrated circuit, the oscillator or clock circuit provided to produce a refresh clock must be sufficiently constant so that the memory cells of the DRAM integrated circuit are refreshed in a timely manner in accordance with design specifications. Hence, it is desirable that the refresh clock have a constant frequency regardless of temperature variations. If the frequency of the refresh clock is faster than necessary, extra energy or power is wasted in refreshing the DRAM integrated circuit too frequently. On the other hand, if the frequency of the refresh clock is too slow, the DRAM integrated circuit can fail and thus lose its stored data. Hence, the conventional oscillator designs produce clocks with frequencies that vary over time and thus tend either waste energy or lose stored data when temperature variations are incurred.
Thus, there is a need for improved oscillator designs that produce oscillators or clocks that are not effected by temperature variations.